The higher olefins oligomerization process converts light olefins, typically, C3 to C6 light olefins, to oligomers (higher olefins), typically such as octenes, nonenes and dodecenes. These higher olefins are then used in the production of various products such as plasticizers and solvents. The feedstocks used for the higher olefins oligomerization process come from various sources, such as catalytic crackers and steam crackers. Such feeds are known to contain nitrogen containing compounds, which act as poisons for the catalysts typically used in the higher olefins oligomerization process. The presence of poisons in the feeds has a significant impact on the catalyst life, and thus on the operation and economics of the higher olefins oligomerization process. It is known that acidic catalysts like solid phosphoric acid or zeolites typically used in olefin oligomerization processes are susceptible to poisoning from trace amounts of sulphur-, nitrogen- and oxygen-containing compounds in the feed. Such poisons adsorb on the acidic catalysts, blocking acid sites and pores. This causes enhanced deactivation of the catalyst and shorter catalyst life. Special precautions and feed cleanup is required in case the poison levels are too high.
At present there is no known single process that can quantitatively remove all nitrogen poisons from olefin feeds useful in the higher olefins oligomerization process to meet required feed quality specifications. Water washing only partially removes nitriles, such as acetonitrile, from certain olefin feeds. Not only is the removal process difficult but it is expensive and generates a lot of waste water.
It is known that commercial guard beds such as Selexsorb CD alumina guard bed can adsorb nitriles from propene, butene and/or pentene-containing feedstreams. Unfortunately, the guard bed capacity is too low to achieve the commercial run lengths, particularly when these commercial feeds contain 5 to 80 ppm acetonitrile and/or propionitrile. A common practice is to regenerate spent guard bed by heating under e.g. a nitrogen flow, or a hydrocarbon flow which is free of nitriles to achieve full desorption form the guard bed. For example EP1216978 mentions isopentane as the desorbing solvent. Adsorption Science & Technology Vol. 23 No. 10 2005 p. 813 to 825 “Options for Nitriles Removal from C4-C5 Cuts: 1. Via Adsorption” by M. M Ramirez-Corrodores et al. discloses use of methanol or pentane as a desorbent. However, such processes require an extra supply of nitrogen or hydrocarbons as well as the corresponding additional apparatus features. This is therefore costly and also proves to be a discontinuous process requiring regular replacement of the nitrogen and hydrocarbon desorbents. Such processes also have the disadvantage of generating waste streams that require special handling.
The interaction of acetonitrile with olefins and alcohols in zeolite H-ZSM-5 is described in Chem. Eur. J. 1997, 3, No. 1 pages 47 to 56 “Interaction of Acetonitrile with Olefins and Alcohols in Zeolite H-ZSM-5: In-Situ Solid-State NMR Characterization of the Reaction Products” Alexander G. Stepanov and Mikhail v. Luzgin.
U.S. Pat. No. 5,414,183 discloses isomerization and etherification reactions. Nitrogen contaminants in the hydrocarbon feed stream are converted to hydrolysis products by contact with an alkaline solution. Residual products in the hydrocarbon phase may be removed by a variety of known means including water washing, stripping and adsorption. Spent adsorbent is regenerated in the vapour phase using a contaminant free stream including one or more of hydrogen, nitrogen fuel gas, natural gas, and clean light hydrocarbons such as propane, butanes, and/or pentanes.
U.S. Pat. No. 4,973,790 discloses a process for oligomerizing C2 to C10 olefins obtained by catalytic cracking of heavy crude oil. Feed pretreatment is practised to remove basic nitrogen compounds present in the light olefin feed with a water wash or guard bed. Ion exchange resins which are used to take out basic nitrogen from the feed only work one time and cannot be regenerated. Zeolitic guard beds may be regenerated by heating. Oxidative regeneration is used for alumina.
US 2005/0137442 relates to a transalkylation process where organic nitrogen compounds, including acetonitrile and propionitrile, are removed from an aromatic feed stream by contacting the stream with an acidic molecular sieve at a temperature of at least 120° C. The adsorption bed may be regenerated with a hot natural gas stream or by a carbon burn.
U.S. Pat. No. 5,744,686 discloses a process for removal of nitrogen compounds from an aromatic hydrocarbon stream. The process includes a fractionation zone and an adsorption zone where the feedstream is passed to the fractionation zone to provide a dry bottoms product stream essentially free of the nitrogen compounds and an overhead stream. The overhead stream is condensed to provide an aqueous stream and a hydrocarbon stream. The hydrocarbon stream is passed to an adsorption zone and a treated effluent recovered therefrom is returned to the fractionation zone.
US2007/0086933 discloses a transalkylation process for reacting carbon number nine aromatics with toluene to form carbon number eight aromatics such a para-xylene. The process uses an alumina guard bed in order to remove chlorides from the aromatic feed prior to contacting with a transalkylation catalyst. The transalkylation effluent is recycled partially to the transalkylation zone or the alumina guard bed zone.